1. Field of the Invention
THIS INVENTION relates to a means of providing a significant increase in the retention of oral appliances designed for the treatment of sleep related disorders, namely snoring and obstructive sleep apnoea (OSAX with consequent health benefits.
2. The Prior Art
Snoring and sleep apnoea are caused by partial or total collapse of the upper airway during sleep. Reduction in muscle tone at the beginning of sleep together with structural factors such as obesity, retro-positioning of the face, retrognathia, tonsillar hypertrophy and macroglossia have been proposed as aetiological factors. There is a wide spectrum of severity from the simple snorer to the patient with very severe sleep apnoea. All patients with obstructive sleep apnoea snore but not all snorers have sleep apnoea. A large proportion of subjects fall into the middle of this spectrum with variable amounts of snoring and OSA.
The creation of a pneumatic stent for the upper airway by means of a facial mask and a calculated pressure of air is a safe and effective method of treatment for both snoring and OSA. This approach to treatment is referred to as nasal continuous positive airway pressure (CPAP) and has for many years been the gold standard for treatment of these conditions. For patients who are simple snorers or those who have OSA with or without excessive daytime sleepiness, CPAP may be an unacceptable method of treatment and other alternatives need to be evaluated. Although compliance with CPAP is generally considered to be high, there is increasing evidence of a lower level of compliance than previously considered with consequent health risk. A surgical approach to the soft palate can be employed for those patients who are simple snorers or who have mild OSA, but this is not considered a safe and effective method of treatment for those patients with moderate or severe OSA, and moreover the long term effectiveness of such procedures has yet to be evaluated.
More recently, oral appliances have been proposed as an alternative to CPAP. Many of these devices depend for their effectiveness on being able to satisfactorily hold the lower jaw and tongue in a forward position during sleep in order to enlarge the airway behind the tongue, and this group of appliances are known as mandibular advancement splints (MAS). In order for the tongue to be able to stay forward, it is important for the tension on the major muscle of the tongue (the genioglossus) and the pharyngeal muscles to be maintained.
Limitations in Terms of Retention of Present Oral Appliances
Visual endoscopic examination during simulated sleep (sleep nasendoscopy) with the wearing of appliances which are designed to hold the tongue and jaw in a forward position shows that if a close relationship of the appliance to the natural teeth is not maintained, even if the displacement occurs to a small degree, the tongue and soft palate drop back in the throat narrowing the airway, and as a consequence this reduces the effectiveness of the appliance. These observations undoubtedly contribute to the results reported in randomized cross-over studies of oral appliances and CPAP where the same level of effectiveness as CPAP is not generally achieved with the oral appliance. The effectiveness of treatment is evaluated in terms of reduction of the so-called respiratory dysfunction index (RCI or AHI-Apnoea hypopnoea index), calculated by comprehensive, complex overnight sleep studies (polysomnography). In addition, the oral appliances as a general rule do not fully maintain the desired hight levels of oxygen saturation in the blood essential to the maintenance of full body function including the brain. For these reasons it has been considered that oral appliances which advance the lower jaw may only be considered an effective method of treatment in some patients with simple snoring and mild and moderate OSA.
All oral appliances used with patients who have their own natural teeth therefore depend for their effectiveness on maintaining that vital apposition with the teeth during sleep. This is achieved by engaging surfaces of the teeth which are below their line of greatest contour to a lesser or greater degree in order to provide resistance to displacement, the so-called “retention” of the appliance. Where the material used for construction of the appliance is of an all enveloping soft resilient material (e.g., polyvinylacetate-polyethylene polymer) or a combination of a harder outer casing with a soft resilient inner lining (poly carbo lactone), the outer surface of the teeth can usually be engaged for retentive purposes. Because the softer material on the inner aspect has the capacity to deform during insertion and removal of the appliance, it can also deform and be displaced during sleep if the pull of the lower jaw during sleep is considerable. If the material used for construction of the appliance is of a relatively harder unyielding nature, e.g., polymethyl methacrylate or metal, it is better capable of holding the lower jaw in a forward position and offering resistance to the forces to which it may be subjected with minimal distortion when compared with a flexible material. With appliances constructed of a metallic cast material, extensions of the base material in the form of dental clasps into the areas below the line of greatest contour afford retention. Where other more rigid non-metallic materials are employed, the material may be extended into the undercut regions in a thinner section so that a degree of flexion in these zones is possible or wrought stainless steel wire extensions are used to form dental clasps for retention. The limitation in terms of the degree of retention which can be obtained with this approach is that only a small proportion of the undercut area available for retention can be usefully engaged (e.g., approximately 10%) due to the essential requirement for the patient to be able not only to insert the appliance without undue difficulty but equally importantly to be able to remove it. The clinical assessment of how much retention has been achieved by this approach to gaining retention is made on a purely arbitrary basis while the patient is conscious. It bears little relationship to the retention and stability during sleep when very large displacing forces can come into effect, not only in terms of attempted jaw opening but also in relation to extreme lateral jaw excursions. As the degree of severity of OSA increases the retention of the appliance becomes more critical which probably accounts for the lack of published success of oral appliances generally for severe OSA on an equally effective basis when compared with CPAP.
Another disadvantage of conventional methods of providing retention with current oral appliances (based on a unidirectional path of insertion and removal) is that untoward forces can be exerted on the surface of existing crowns, bridges and other dental restorations during insertion, removal and displacement during sleep which can result in their loss from the teeth or damage with much discomfort to patient and dentist alike. From these observations it can be concluded that that the ideal method of gaining retention for an appliance for sleep related disorders would include a method of insertion and removal which permits a passive relationship with the supporting teeth, ease of insertion and removal yet at the same time maintains that necessary vital apposition to the teeth in order to resist displacing forces during sleep.